Overview
The Science of Light has gone all the way from a mere transfer of energy to the creation of colors for entertainment and other purposes. Most of our gadgets are light emitting for efficiency when used at night. In the field of medicine light is used to cut through the skin for surgery as in laparoscopy. The health sciences are also using light for other medical purposes. But the most important purpose is for humans and other animals to see the beautiful world through light. So, are you ready to explore the characteristics and properties of light?
In this module, you will learn some properties and characteristics of light.
Among the characteristics and properties of light, we will focus on refraction and specifically, dispersion of light. We will try to find through simple activities how light disperse to form the colors of light. We will also try to find the hierarchy of colors of light in terms of frequency, wavelength, and energy. The different activities provided in this module will make us realize the beauty of everything with light.
How are refraction and dispersion demonstrated in light?
Among the different colors of light, which is bent the most and the least?
Why do we see spectacular events in the sky like rainbows, red sunset and blue sky?
Unit 1
MODULE
6
Suggested time allotment: 6 to 8 hours
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Refraction of Light
Did you know that the boy made the stunt in a 6-ft deep swimming pool? But as it appears in Figure 1 the water is just shallow and the stunt would not be dangerous at all. This optical illusion is known as apparent depth. Apparent depth is the illusion that objects under the water appear to be nearer the surface than they really are. This is visible when an observer is standing beside the swimming pool looking at an object under water. This phenomenon is a consequence of the bending of light when light traverses the air-water boundary.
Figure 1. Apparent depth β¦ Refracted light
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Refracting Light
Light exhibits the characteristics and properties of a wave. It is classified as an electromagnetic wave located between the spectrum of infrared and ultraviolet.
As an electromagnetic wave it does not need a medium in order to propagate. It moves in its maximum speed in vacuum. But this speed decreases as it moves along different media. This characteristic of light consequently shows bending when it crosses the boundary between two media. Apparent distortion of an object seen at the boundary between media is observed.
Figure 2. The Electromagnetic Spectrum 103
1 kilometer 100 1 meter
10-3
1 millimeter 10-6
1000 nanometer 10-9
1 nanometer 10-12 meters
700 nanometers 600 nanometers 500 nanometers 400 nanometers
Long Wavelengths Short Wavelengths
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Figures 3 and 4 are the basic examples of refraction of light. Refraction is the bending of light when it travels from one medium to another of different optical densities. The pencil as shown in Figure 4 is not really broken. If we remove the water from the glass and look at the pencil, the pencil would look normally straight.
Now try pouring water onto the glass and, voila - a broken pencil. This happens because of the change in speed and orientation of the light with respect to the normal as it traverses a new medium of a different density.
Figure 3. Show me the coinβ¦
Figure 4. Broken pencil
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Light travels so fast. From your lesson last year, it is approximated to travel at a speed of 3 x 108 m/s in a vacuum. This speed decreases when light travels in a dense medium. This means that the speed of light is dependent on the properties of the medium. In the case of light, it is dependent on the optical density of the medium.
The optical density of the medium is different from its physical density. Physical density is described as the mass per unit volume of the medium. On the other hand, the sluggishness of the atoms of a medium to maintain the absorbed energy before reemitting it is called optical density. When light crosses the boundary of two media of different optical density, a change in speed takes place. This change in speed is manifested as bending of the light ray.
Figure 5. Refraction of light
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In Figure 6, light travels from air to water. We observe that the incident angle (<i) is greater than the angle of refraction (<r). We can see that the light ray refracts or bends towards the normal. Thus, light bends towards the normal when traveling from a less dense medium to a higher density medium.
A known indicator of the optical density of a material is the index of refraction of the material. Index of refraction represented by the symbol n is the ratio of the speed of light in vacuum and its speed in another medium. In symbols;
π = π ππππ ππ πππβπ‘ ππ π£πππ’π’π π ππππ ππ πππβπ‘ ππ πππ‘πππππ= πΆ
π£
Figure 6. Refraction of Light in Water
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The index of refraction of a material is a quantity that compares the speed of light in that material to its speed in a vacuum. Since the speed of light in vacuum is the highest attainable speed in the universe, the index of refraction is always greater than 1. The n values of other media are shown in Table 1.
Table 1. Index of refraction of other materials Materials Index of Refraction
Diamond 2.147
Zircon 1.923
Light flint glass 1.580
Crown glass 1.520
Ethyl alcohol 1.510
Water 1.360
Ice air 1.310
Vacuum 1.000
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